After living on the island for 23 years, what does robinson crusoe say keeps him from being the happiest man in the world?

It was "his fear of savages".

Daniel Defoe wrote this spectacular tale which was published in 1719. When the book was first published many people who read it actually believed that the tale is told by a traveler who actually went through all of the experience. The popularity can be estimated by the fact that before the end of 1719 it had already gone through four editions.